Oxide film resistor



Spt, 23, 1969 G. cANEGALLo OXIDE FILM RESISTOR Filed Jan. 3, 1967 FIGQ, 13.

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United States Patent O 3,469,227 OXIDE FILM RESISTOR Giovanni Canegallo, Milan, Italy, assignor to Welwyn Electric Limited, Bedlington, England, a corporation of Great Britain Filed Jan. 3, 1967, Ser. No. 607,058 Claims priority, application Italy, Jan. 11, 1966,

13,290/ 66 Int. Cl. H01c 1/02 U.S. Cl. 338-262 4 Claims ABSTRACT F THE DISCLOSURE A substrate has an oxide resistor thereon. A protective metal oxide covers the resistor film except at the end portions wherein metal regions blending into the protective film are placed. Terminals are placed at these metal regions.

It is well known that oxide film resistors require to be protected against deterioration caused by outside influence, e.g. adverse atmospheric conditions due to humidity or other causes.

Most methods of protecting oxide film resistors employ a relatively impervious organic or inorganic material applied over the resistive element which is constituted by an oxide film. In the majority of cases metallic terminals are first fitted to the resistors to make contact with the resistive film and then the insulating material, often in the form of a lacquer or a cement, is lapplied over the resistor film and over the metallic terminals, thus resulting in some form of discontinuity at the point Where the lacquer covers the junction between the terminal and the resistive film. It has further been proposed in U.S. Patent No. 2,915,730 to use as protective materials certain metal oxide films, which consist of tin oxide with another metal oxide, the purpose of which is to decrease the conductivity of the tin oxide, thus allowing the terminals to be applied over the protective films. The deposition of these non-insulating films have disadvantages in that one has to carefully control the resistivity of the protective film in order to obtain conductivity which will not be too low to give a poor contact between the terminals and the film, but not too high to lead to interference with the resistance value by parallel conduction in the protective film; at best, even with extremely accurate control, these particular non-insulating films can only provide a compromise solution.

It is the purpose of this invention to provide an elec. trical oxide film resistor with a protective film which provides protection over the resistance element while giving a good contact at the terminal regions and avoiding any discontinuity at these points.

The present invention provides an electrical resistor comprising a body of insulating material having adhering thereto an electrical resistance element of an oxide film, a protective film superimposed thereon and metal terminals, said protective film consisting of a continuous layer of metal oxide merging into terminal regions of metal corresponding to said metal oxide, said metal terminals being in electrical contact with the metallic regions of the protective film, said metal oxide film portion having an electrical conductivity of a lesser order than that of the electrical resistance element or being electrically insulating. It should be noted that the terminals of the resistor of the invention need not be in the form of conventional metal caps; they can take any `desired form or shape and, eg., may even take the form of a layer of solder.

3,469,227 Patented Sept. 23, 1969 The electrical resistor of the present invention is preferably constructed in such a way that the protective film consists of copper oxide, iron oxide or magnesium oxide merging at the terminal regions into copper, iron or magnesium respectively.

In order to provide even better contact between the metal terminals and the terminal regions of the protective film it is preferred to deposit on the metallic portions of the protective film another metal which should be more inert to oxidation than the metal corresponding to the metal oxide forming the protective film. This said second metal may be nickel or cobalt. However, other inert and not easily oxidisable metals may be used for this purpose, for example noble metals, e.g. silver, gold or platinum. This second metal could additionally be deposited on the terminal regions directly on the oxide lm resistance element.

The invention further provides three methods of producing resistors having a protective film of a layer of metal oxide merging into metal.

The first method of providing the protective film in accordance with the invention comprises depositing adherently on an electrical oxide film resistor a layer of met-al capable of being oxidised under conditions substantially not adversely affecting the electrical oxide film resistance element, applying a masking material to part of the resulting metal film leaving terminal regions free of masking material, plating the unprotected terminal regions with a metal less easily oxidised than said first metal, removing the masking material and oxidising the areas of the said first metal protective film previously covered by masking material without substantially adversely affecting said resistance element. Since in this method there is no need to reduce metal oxide at the terminal regions, in addition to copper and iron which can also be used yfor the second method of the invention as will be described hereinafter, magnesium (which does not form an easily reducible oxide) may be used for forming the protective film.

In the first method of the invention plating with a second lmetal the terminal regions of the protective film is essential, but this procedure is also preferable in the second method to be described hereinafter, suitable metals for this purpose being as indicated above.

The second of these three methods comprises depositing adherently on an electrical oxide film resistor a layer of a metal formingv a reducible oxide and capable of being oxidised under conditions substantially not adversely affecting the electrical oxide film resistance element, oxidising the resulting metal film without substantially adversely affecting said resistance element, applying a masking material to part of the resulting oxidised film leaving unmasked terminal regions, reducing the exposed metal oxide without thereby substantially adversely affecting said resistance element and removing the masking material. Preferred metals for use in this method are copper and iron; these two metals are not only capable of being oxidised under conditions which do not adversely affect the electrical oxide film resistance element (this usually comprises tin oxide), but copper oxide and iron oxide are reducible under comparatively mild conditions to which the resistors may lbe exposed without deleteriously affecting the resistance film.

Suitable materials for masking the protective film in both the above methods of the invention include inert organic lacquers which provide good protection during the metallising or plating processes and yet are easily removed. Examples include vinyl or nitrocellulose lacquers as are commonly used for stopping-off purposes.

It will be noticed that all the above specific metals for forming the protective -lm are ones which can be oxidised at a temperature below 600 C.

In effecting the above two methods of the invention the amount of metal deposited for protective film formation should be such that the eventual metal oxide protective film resulting after oxidation has a low conductivity, i.e. its conductivity should be of an order less than that of the resistance element of the film resistor. In general, it should have a resistivity greater than l megohm/ square. In this way the protective layer of metal oxide acts as a very effective protection against mechanical and chemical damage but does not Substantially affect the total resistance value of the resistor. The resistance value of the protective layer may be easily controlled by regulating the thickness of the coating deposited. For example, in the case of copper the initial thickness deposited on the oxide resistance element may be 100 Angstrom units to 10,000 Angstrom units or more to give a satisfactory copper oxide coating after oxidation.

The adherent deposition of the oxidisable metal film from which the protective coating is formed may be effected by electrodeposition, electroless deposition, Vacuum deposition or any other of the well known techniques for the deposition of such a lm.

The resistance element of the resistors of the invention is normally formed of tin oxide with or without one or more other oxides but it is within the scope of the present invention to use other oxide film resistance elements, e.g. cadmium or titanium.

The present invention also provides a third method for making the above resistors. This third method consists of a process for the production of an electrical resistor having a protective film of a layer of metal oxide merging into metal, which comprises depositing adhercapable of being oxidised under conditions substantially not adversely affecting the electrical oxide film resistance element, applying metallic electrical terminals on the resulting metallic film and oxidising that portion of the metallic film remaining exposed without substantially adversely affecting the electrical oxide film resistance element. The preferred metal for this purpose is copper.

It should be noted, however, that this third method of making the resistors of the present invention does not allow the making of useful electrical measurements in the case of automatic production prior to fitting the metallic terminals.

In the third method of the invention the electrical terminals are formed to an appropriate shape to make electrical contact to the ends of the resistor and fabricated in any metal or alloy having the necessary mechanical and electrical properties and which will withstand the conditions occurring during the oxidation of the metal, e.g. copper, without suffering any substantial deterioration. These terminals would normally take the form of a cap having an internal diameter somewhat less than that of the external diameter of the resistors, so as to produce a firm interference fit when forced on to the rod. The internal diameter of the cap is chosen so that it can be forced on to the resistor without the use of excessive pressure which would otherwise damage or remove the deposited metal film, or damage the resistor or metal terminal. The terminal is of a form and material such that at a later stage in the process, preferably after the subsequent oxidation process, wire leads are fitted by soldering or welding. A material which has been found to be suitable for use in the form of metallic caps is nickel plated mild steel.

When copper is the oxidisable metal used in the methods of the invention, oxidation results in a copper oxide film which has a redbrown to grey and even black aspect, thus indicating various stages of oxidation.

The invention will now be further described Iby reference to the accompanying diagrammatic drawings in which:

FIGURE 1 shows a cross sectional view of an embodiment of a resistor constructed in accordance with the invention in the course of its manufacture by the first method of the invention;

FIGURE 2 shows a cross sectional view of 'the resistor of FIGURE l during another stage of its manufacture;

FIGURE 3 shows a cross sectional view of an embodiment of a resistor constructed in accordance with the invention in the course of its manufacture by the second method of the invention;

FIGURE 4 shows a cross sectional view of an embodiment of a resistor constructed in accordance with the invention in the course of its manufacture by the third method of the invention; and

- FIGURE 5 shows a cross sectional view of the resistor of FIGURE 4 after terminals and leads have been fitted.

Referring first to FIGURE 1, a chemically inert organic masking lacquer 12 is applied to metal film 11 in such a way that the areas where terminals are to be applied are left unprotected. These areas are plated with a metal 13 which is more inert to oxidation than the metal layer 11. The masking lacquer is then removed and the resistor heated in an oxidising atmosphere such that area 21 (see FIGURE 2) between the terminal bands becomes fully oxidised, leaving terminal bands 22 substantially unaffected and therefore still possessing good electrical conductivity.

The above description relates to the use of the first method of the invention. The following description refers to the second method of the invention.

Referring now to FIGURE 3, a metal film over the whole of the resistor surface is oxidised by heating in a plentiful supply of air to produce a metal oxide film 31. An organic masking lacquer 32 is applied to the oxidised layer, leaving those areas exposed Where terminals are to be applied. These exposed areas are then subjected to reducing conditions, thus creating highly conducting terminal bands which can then, if required, be plated with another metal having high conductivity and being resistant to oxidation, e.g. by an electro or electroless plating process.

Referring to FIGURE 4 which refers to the third method of the invention, a resistor has been coated with a film of copper 41 and metallic end caps 42 applied thereto without damage or removal of the copper film.

Referring to FIGURE 5, the resistor of FIGURE 4 has been oxidised so that the exposed portion 51 of the copper film has become oxidised, while the protected portions 52 remain substantially unaffected due to the presence of the tightly tting metallic end caps 53. Wire leads 54 are attached to the metallic end caps.

There are many metals which have a sufficiently high rate -of oxidation in the temperature range suitable for them to be used for the protective insulating oxide layer. The following table gives the thickness of the oxide layer produced on just a few metals when heated at 500 C. for 1 hour.

Thickness of oxide layer Metal: in Angstrom units Magnesium 10,000 Iron 5,000 Copper 1,600

On the other hand, under similar conditions nickel and cobalt are not appreciably oxidised.

The figures quoted in the above table are based on information for the bulk metals and serve only as a guide for comparison purposes. Electroless films are known to be somewhat more open in structure and oxidised at a faster rate.

The following examples illustrate the invention without limiting it. Copper was chosen for the oxidisable metal because of its ease of application by a non-electrolytic process.

EXAMPLE 1 (a) Copper deposition Sensitiser:

Stannous chloride g-- 10.0 Methyl alcohol cc 60 Hydrochloric acid (concentrated) cc-.. 25 Distilled water cc 1000 Activator:

Palladium chloride -g 1.0 Hydrochloric acid (concentrated) cc 3.0 Distilled water cc-- 1000 Ceramic substrates carrying resistance elements of tin oxide iilm manufactured in known manner, e.g. as de-y Plating solution Solution 1:

Copper sulphate hydrated g 34.6 Distilled water cc-- 500 Solution 2:

Sodium potassium tartrate g 173.0 Sodium hydroxide g-- 52.0 Distilled water cc-- 500 In order to illustrate the process in more detail, the plating of 1000 tin oxide coated ceramic rods, 2.5 mm. diameter and 8 mm. long, will be described. These sensitised and activated rods are placed in 45 cc. of the mixed plating solutions containing 40 g./ litre of trioxymethylene as reducing agent and agitated during a 10 minute plating period at 20 C., thus ensuring a uniform deposit. The excess plating solution is then poured off and the rods are thoroughly rinsed in running tap water, distilled Water, and finally acetone. These conditions produce a copper film- 5000 Angstrom units in thickness. For a given volume of plating solution the thickness of the copper deposit can be varied by varying the time and temperature of plating. Film thicknesses varying from 100 Angstrom units to 10,000 Angstrom units have been found to be satisfactory. This point is illustrated by reference to the following table, which refers to the approximate thickness of metallic copper deposited on rods of 2.5 mm. diameter and 8 mm. in length.

COPPER DEPOSITION 0N CERAMIC RODS [2.5 mm. diameter and 8 mm. long] (b) Formation of oxide after plating terminal regions An organic masking lacquer consisting of vinyl or nitrocellulose lacquers as normally used for stoppingoif is applied to the rods, leaving termination bands exposed at the ends. In this case a commercially available vinyl lacquer known as Bitulac VB 3106/70 obtainable from Bitulac Limited of Newcastle-upon-Tyne, England is used. These termination bands are now nickel plated by an electroless method using a known process and using the following meallising solution.

6 Nickel plating solution:

Nickel chloride g 28.0 Sodium hypophosphite g-- 17.0 Sodium citrate g-- 50.0 Ammonium chloride g 80.0 Distilled water cc-.. 1000 The nickel deposition is effected by a similar method of the copper deposition process; however, the copper surface, on which the nickel is to be plated, does not require sensitising or activating and the nickel plating process is carried out directly on the copper surface without any further treatment. The 1000 copper plated and masked rods are placed into 200 cc. of the plating solution, 40 cc. of 20% sodium hydroxide solution are added and the solution heated almost to the boil (99 C.). It is held at this temperature for 15 minutes, the solution is then poured oi and the rods thoroughly washed in water. They are finally rinsed in a water soluble organic solvent. This organic solvent will depend on the nature of the particular lacquer used, but would commonly be acetone, ethyl acetate or some suitable mixture of ketones and esters; in this instance the commercially available solvent for said Bitulac is used. It is chosen since it is able to remove both the water and the organic masking material. The deposited nickel has a surface resistivity of about 0.1 ohm per square and is about 50,000 Angstrom units in thickness. The rods are then placed in containers made of some suitable material which will not contaminate or otherwise impede the process of oxidation. Aluminum trays have been found to be suitable for this purpose. The rods are then heated in a static or continuous oven for a suitable time at a temperature suiiicient to achieve substantial oxidation of the copper lm. It has been found that the temperature would generally have to exceed C. and in the particular case of a copper film of thickness 5,000 Angstrom units a period of two hours at 400 C. is suflicient substantially to oxidise the copper leaving the nickel terminal bands almost unaffected. The atmosphere in the oven can be of any type capable of oxidising the copper such as still or circulating air, air enriched with oxygen, pure oxygen, ozone or certain oxides of nitrogen. ln the case of a lm of copper of thickness 5,000 Angstrom units substantially oxidised in this manner, the resistivity of the resulting film will now be in the region of 8 megohms per square.

EXAMPLE 2 (a) Copper deposition Copper deposition was effected as indicated in Example 1(a).

(b) Oxidation of the copper coating The copper plated rods are oxidised as described in Example 1(b).

The rods having the copper oxide protective film thereon are then masked with an organic masking lacquer consisting of vinyl or nitrocellulose lacquers as commonly used for stopping-off purposes, leaving the terminal areas exposed. 1000 rod shaped lm resistors similar to the ones described in Example 1 are plated in 45 cc. of the mixed copper plating solution having the composition stated in Example 1; agitation is eifected at 20 C. during the plating period which lasts l0 minutes. During this plating process the early stages are concerned with the reduction of the copper oxide on the termination area by the reducing agent, trioxymethylene. This is followed by the deposition of additional copper on to these areas from the solution. After the reduction and plating, the rods are thoroughly rinsed in running tap water and either further plated with a more inert layer of electroless nickel as described in Example 1 or they can be rinsed in distilled water, then acetone, and dried, giving a rather more reactive copper termination area.

It should be noted that in the above two examples there is a stage in the manufacturing process at which the tin oxide film resistors are provided both with a protective layer of metal oxide in the central regions of the resistor and metallised terminal bands at the ends. It is therefore possible to make excellent electrical connection to the resistor by means of the metallised terminal bands before the usual metallic terminals (or caps) are added to the metallised bands. This is a most valuable feature of the invention as it facilitates the automatic processing of resistors and allows initial grading into resistance value, helical cutting of a groove to achieve higher and accurately determined resistance Values while the resistor still has a substantially smooth cylindrical contour and before metallic terminals or their associated wire leads have been attached. It is always much more diiicult'to process on automatic machinery resistors with metallic terminals and leads fitted thereto. In addition, an economic advantage ensues in that, by carrying out preliminary electrical measurement and grading according to initial resistance value, those resistors having values outside the tolerance permitted for the maintenance of adequate quality in the process can be rejected before any further processing such as the fitting of metallic terminals is carried out and before the cost of said metallic terminals is added to that of the initial resistor. It should be noted that a further advantage turns on the fact that, when the metallic terminals are fitted on top of the metallised bands, there is no substantial change to the resistance value.

EXAMPLE 3 (a) Copper deposition Copper deposition is effected on substrates similar to those described in Example 1(a), using a similar method. The copper film thickness in this example is 2000 Angstrom units.

(b) Formation of oxide after fitting electrical terminals The copper plated substrates are fitted with electrical terminals in the form of metallic end caps. Each cap is in the shape of a cylinder with one closed end formed by cold forming sheet mild steel which is then nickel plated. The steel is 0.008 thick, the internal diameter of each cap is 0.094 and its length is 0.053. The mild steel is electro-plated with 1 to 2 tenths thou. of nickel. The rod is 0.098 diameter and 0.315 long. By this method of applying the metallic terminals care is taken that they fit the resistor bodies very closely and afford adequate protection to the underlying copper iilm'so as to avoid oxidation thereof during the subsequent procedure. The resulting resistors are placed in a static or continuous oven for 1 hour at a temperature of about 300 C. in air. Instead of using pure air it is likewise possible to use air which has been enriched with oxygen or pure oxygen, ozone or certain oxides of nitrogen. In this way the copper film not covered by the metallic terminals is converted to copper oxide, but the copper film covered by these terminals is left substantially unchanged. The fact that the copper film under the terminals remains unoxidised is confirmed by values obtained on measuring current noise? and non-linearity of the resistors. Both of these parameters are a measure of the quality of the contact which is obtained with the resistance element on the substrates, i.e. particularly low values were obtained for these two parameters.

It should be noted that in all three methods of making the resistors of the present invention the level of heat treatment which is required to oxidise the deposited metal films is much lower than that which would affect the underlying resistive oxide film, and furthermore, the metallic oxide constituting the protective film need not necessarily be electrically conductive.

Although the present invention is described herein with particular reference to specific details, it is not intended that such details shall be regarded as limitations upon the scope of the invention except insofar as included in the accompanying claims.

I claim:

1. An electrical resistor comprising a body of insulating material having adhering thereto an electrical resistance element of an oxide film, a protective film superimposed thereon and metal terminals, said protective film consisting of a continuous layer of metal oxide merging into terminal regions of metal corresponding to the last-named metal oxide, said metal terminals being in electrical contact with the metallic regions of the protective film, said metal oxide film portion having an electrical conductivity of a lesser order than that of the electrical resistance element.

2. A resistor according to claim 1, in which the protective film consists of copper oxide, iron oxide or magnesium oxide merging into copper, iron or magnesium respectively.

3. A resistor according to claim 1, in which the metallic portions of the protective film have deposited thereon another metal more inert to oxidation than the first-mentioned metal.

4. An electrical resistor comprising: a body of insulating material having adhering thereto an electrical resistance element in the form of an oxide film; a continuous protective lm adhering to said resistance element, said protective film consisting at its terminal ends of a metal portion in contact with said resistance element and consisting intermediate its ends of an oxide portion formed of an oxide of said metal, the oxide portion and metal portion of said protective film merging and blending into each other without abrupt change, said merging and blending being a result of one of said portions having been formed in situ from an initially homogeneous film, and said oxide having an electrical conductivity less than that of said resistance element; and a metal terminal in electrical contact with each of said metal portions of said protective film.

References Cited UNITED STATES PATENTS 2,934,736 4/1960 Davis 338-308 E. A. GOLDBERG, Primary Examiner U.S. Cl. X.R.

Patent No. 3,469 ,227 September 23 1969 Giovanni Canegallo error appears in the above identified It is certified that nt are hereby corrected as patent and that said Letters Pate shown below:

Column 3, line 35 after "adher" insert ently on an electrical oxide film resistor a layer of a metal Column 4, line 7l, "oxidised" should read oxidise Column 5, line 75, "meallising" should read metallising Column 6,

line 8, "of" should read to Signed and sealed this 27th day of October 1970.

(SEAL) Attest:

WILLIAM E. SCHUYLER, J

Edward M. Fletcher, Jr.

Commissioner fof Pateni Attesting Officer 

